AN1902 Freescale Semiconductor / Motorola, AN1902 Datasheet - Page 13

• Manufacturer Part Number: AN1902
• Description: Quad Flat Pack No-Lead (QFN)
• Manufacturer: Freescale Semiconductor / Motorola

Freescale Semiconductor, Inc.
AN1902/D
11.8mils (0.30mm) separation between the openings. For the 9x9 QFN, the recom-
mended opening dimension is 2mm x 2mm, 21.7mils (0.55mm) apart.
2.0mm
0.85mm
0.3mm
0.55mm
Figure 15 (L) 4x4mm, (R) 9x9mm Stencil Array Design from Motorola MAP QFN Study
4.2.2 Additional Recommendation from Motorola MAP QFN Study
4.2.2.1 Aspect and Area Ratios
Aspect and area ratios are two important parameters in the solder paste printing pro-
cess. These two ratios determine the amount of solder paste transferred from the
stencil to the PCB pad. Aspect ratio is defined as the ratio of aperture width to stencil
thickness. Area ratio is complicated to define in text, but the equation is provided in
Table
5. For a good solder paste printing process, keep the aspect ratio larger than
1.5 and the area ratio larger than 0.6. Too small of a ratio will cause the solder paste to
stick on the side wall of the stencil aperture instead of the land of the PCB.
MOTOROLA
Quad Flat Pack No-Lead (QFN)
13
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